This invention relates to devices and methods for establishing magnetic field gradients, and in particular to etched Z-axis magnetic field gradient coils for magnetic resonance applications such as nuclear magnetic resonance spectroscopy and magnetic resonance imaging.
Nuclear Magnetic Resonance (NMR) and magnetic resonance imaging (MRI) devices often employ a superconducting magnet for generating a static magnetic field B.sub.0 along the z-axis, and a special-purpose z-axis gradient coil for generating a magnetic field gradient along the z-axis. It is common notation to label the other two cartesian axes the x- and y-axes, and the angle measured from the x-axis in the x-y plane.
In one prior art approach, a z-axis gradient coil is made by winding wire coils on a cylindrical support, with a predetermined current distribution chosen to generate the desired magnetic field gradient. The predominant direction of the windings is then in the transverse (x-y) plane, with a relatively small component along the z-axis needed to make connections between adjacent turns. The magnetic field generated by the z-axis component of the windings can be canceled out by winding a second layer of turns in the same angular direction but opposite z-direction. While wire coils are capable of generating satisfactory magnetic field gradients, such coils are often relatively expensive to fabricate.
In U.S. Pat. No. 4,910,462, Roemer et al. describe a z-axis gradient coil made by an etched-circuit technique: a conductive pattern is etched onto a flat, flexible sheet and the sheet is rolled into a cylinder. In one suggested design approach (shown in FIG. 2 of Roemer et al.), the coil comprises a two-layer interconnector section that bends back sharply to lie along the cylindrical part of the coil. The sharp bend can generate stresses in the coil material.
Moreover, the interconnector section forms a bulge on the outside of the coil. In another design approach (illustrated in FIGS. 3, 3a and 3b) of Roemer et al.), a two-layer interconnector section lies along the cylindrical part of the coil, without bending sharply. In this approach, however, the z-axis current components in the two interconnector layers have unequal magnitudes, and unwanted current components in the interconnector section are not completely canceled.
In U.S. Pat. No. 5,623,208, Hasegawa describes a z-axis etched-circuit gradient coil formed by winding multiple turns of a flexible substrate on a cylindrical bobbin. As shown in FIG. 2 of Hasegawa, the coil includes a sharp 180.degree. bend along a marginal interconnector region.